Japanese Patent Application No. 2001-66612, filed on Mar. 9, 2001, and Japanese Patent Application No. 2002-60135 filed on Mar. 6, 2002 are herein incorporated by reference in their entirety.
The present invention relates to a surface-emitting type of light emitting device using electro-luminescence (EL), and a display device and an electronic instrument using the light emitting device.
In an EL light emitting element using EL, isotropic light emission deteriorates the directivity so that the intensity of a light in a specific direction is low, and the emitted light cannot be utilized at high efficiency.
The present invention may provide a light emitting device that is superior in wavelength selectivity of light to be emitted in a direction intersecting a substrate and capable of efficiently utilizing light.
The present invention may also provide a display device and an electronic instrument using the light emitting device.
According to one aspect of the present invention, there is provided a light emitting device having a substrate and a light-emitting section provided on the substrate, and emitting light in a direction intersecting the substrate,
wherein the light-emitting section comprises:
a light-emitting layer in which light is generated by electro-luminescence;
an electrode used to apply electric charges to the light-emitting layer; and
first and second dielectric multi-layered films between which the light-emitting layer is interposed; and
wherein the electrode is disposed to avoid overlap with at least part of a light-emitting region in the light-emitting layer, as viewed from a light emitting direction.
The first and second dielectric multi-layered films of this light emitting device enables to improve the wavelength selectivity. As a result, the light generated in the light-emitting layer by electro-luminescence emits at high efficiency in a narrow wavelength band corresponding to a high reflectance band of the first and second dielectric multi-layered films. In addition, since the electrode is disposed to avoid overlap with the light-emitting region in the light-emitting layer as viewed from the light emitting direction, absorption or scattering of light due to the electrode can be reduced. Therefore, the light generated in the light-emitting layer can be utilized at high efficiency.
xe2x80x9cThe electrode is disposed to avoid overlap with the light-emitting region in the light-emitting layer as viewed from the light emitting directionxe2x80x9d means to dispose the electrode so that the light generated in the light-emitting region of the light-emitting layer is not obstructed by the electrode as much as possible during transmission of the light between the first and second dielectric multi-layered films. More concretely, the electrode is not provided in a place overlapping the entirety or part of the light-emitting region as viewed from a direction in which the dielectric multi-layered films are piled (or a direction intersecting a surface of the substrate).
The present invention has various features as follows.
(A) As the electrode, the light emitting device may comprise: a first electrode formed of a pair of electrode layers used to apply electrons to the light-emitting layer; and a second electrode formed of a pair of electrode layers used to apply holes to the light-emitting layer. In this configuration, since the first and second electrodes respectively comprise a pair of electrode layers, one of a pair of the electrode layers can be used as a so-called source, while the other can be used as a so-called drain. The xe2x80x9csourcexe2x80x9d is a region which supplies electrons and the xe2x80x9cdrainxe2x80x9d is a region which supplies holes, for example. Accordingly, holes can be led to a hole transport layer from the drain to the source by applying a voltage so that the drain has positive potential to the source. Similarly, electrons can be led to an electron transport layer from the source to the drain by applying a voltage so that the source has negative electric potential to the drain.
(B) The first electrode may be connected to an electron transport layer and the second electrode may be connected to a hole transport layer This configuration enables to apply a predetermined voltage to the electron transport layer by using the first electrode, and apply a predetermined voltage to the hole transport layer by using the second electrode. Since voltage can be separately applied to the electron transport layer and the hole transport layer, a large number of electrons and holes can be transported.
(C) The light emitting device may further comprise: a third electrode disposed to interpose an insulating layer between the first electrode and the third electrode; and a fourth electrode disposed to interpose another insulating layer between the second electrode and the fourth electrode. The third and fourth electrodes may have a function as a gate for the first and second electrodes. For example, by applying a predetermined voltage to the third and fourth electrodes, the electron transport layer and the hole transport layer can be made to have different potentials, resulting that a lot of electrons in the electronic transport layer and a lot of holes in the hole transport layer can be moved into the light-emitting layer. Thus, the number of the electrons and holes contributing to light emission in the light-emitting layer can be increased, and the light emission efficiency can be raised.
(D) The first and second electrodes may be disposed as follows.
First, the first electrode may be disposed on one side of the light-emitting layer and the second electrode may be disposed on the other side of the light-emitting layer, in the direction intersecting the substrate or the light emitting direction.
Second, the first electrode may be disposed on one side of the light-emitting layer and the second electrode may be disposed on the other side of the light-emitting layer, in a direction parallel to a surface of the substrate.
(E) A wavelength band of a light reflected on the first and second dielectric multi-layered film may be included in a wavelength band of a light generated in the light-emitting layer. In this configuration, the first and second dielectric multi-layered films can form a so-called optical resonator. The light is emitted from a film having lower reflectance in the pair of the dielectric multi-layered films.
The light emitting device according to the present invention can be applied to a display device. This display device can be applied to various types of electronic instruments. Alternatively, the light emitting device according to the present invention can be applied to various types of electronic instruments. Examples of such display device and electronic instruments will be described later.
Now, part of examples of materials usable for each component of the light emitting device according to the invention will be described below. It should be understood that these materials are part of known materials, and it goes without saying that materials other than these examples can be similarly used. Light-emitting Layer
A material for the light-emitting layer is selected among well-known chemical compounds in order to obtain the light having a predetermined wavelength. It may be either of organic and inorganic compounds, but preferably, is an organic compound in view of a profusion of kinds and a characteristic of forming a film.
It is possible to use such organic compound as aromatic diamine derivative (TPD), oxadiazole derivative (PBD), oxadiazole dimer (OXD-8), distyryl arylene derivative (DSA), beryllium-benzquinolinol complex (Bebq), triphenyl amine derivative (MTDATA), rubrene, quinacridone, triazole derivative, polyphenylene, polyalkylfluorene, polyalkylthiophene. azomethine zinc complex, porphyrin zinc complex, benzoxazole zinc complex and phenanthroline europium. which are disclosed in, for example, the Japanese Patent Application Laid-Open No. 10-153967.
More concretely, the material of the organic light-emitting layer to be used can be such well-known material that is disclosed in the Japanese Patent Application Laid-Open No. 63-70257, the Japanese Patent Application Laid-Open No. 63-175860, the Japanese Patent Application Laid-Open No. 2-135361, the Japanese Patent Application Laid-Open No. 2-135359, the Japanese Patent Application Laid-Open No. 3-152184, the Japanese Patent Application Laid-Open No. 8-248276 or the Japanese Patent Application Laid-Open No. 10-153967. These compounds may be used single or in combination of two or more kinds.
ZnS:Mn (in red area), ZnS:TbOF (in green area), SrS:Cu, SrS:Ag and SrS:Ce (in blue area) are exemplified as the inorganic compound.
Dielectric Multi-layered Film
In the light-emitting section, the dielectric multi-layered film has a structure where materials whose refractive indices are different each other are piled alternately. In such a piled layer structure, a layer of silicon oxide (SiO2) and a layer of silicon nitride (SiNx), for example, are piled alternately. Alternatively, two layers selected among TiO2, Ta2O5, MgF2 and ZnS, for example, may be piled alternately to form the dielectric multi-layered film.
Electrode Layer
An electron applying type of metal, an alloy and an electricity conductive type of compound, which have a small work function (not more than 4 ev, for example), and compound thereof may be used for a cathode, that is, an electrode for applying electrons to the light-emitting layer. Such electrode material to be used may be the one disclosed in the Japanese Patent Application Laid-Open No. 8-248276, for example.
A metal, an alloy and an electricity conductive type of compound, which have a large work function (not lower than 4 eV, for example), and compound thereof may be used for an anode, that is, an electrode for applying holes to the light-emitting layer. In the case that an optically transparent material is used as the anode, a conductive transparent material such as CuI, ITO, SnO, and Zno can be used. When the transparence is not required, a metal such as gold can be used.
It is preferable that the following relation is established for the first electrode connected to the electron transport layer:
Work function of one electrode layer (source) greater than Lowest unoccupied molecular orbital (LUMO) of the electron transport layer less than Work function of the other electrode layer (drain)
Furthermore, it is preferable that the following relation is established for the second electrode connected to the hole transport layer:
Work function of one electrode layer (source) less than Highest Occupied molecular orbital (HOMO) of the hole transport layer less than Work function of the other electrode layer (drain)
The third and fourth electrodes are not especially limited so long as they are conductive layers having a function as the aforementioned gate for the first and second electrodes. When the third and fourth electrodes form a dielectric multi-layered film, however, it is required that their materials satisfy the characteristic of the film.
Hole Transport Layer
A material for the hole transport layer provided in accordance with requirements may be selected for use from materials used as a hole applying material for a well-known optical conductive material or from well-known materials used for a hole applying layer of an organic light emitting device. The material for the hole transport layer has one of functions for applying holes and for providing an electron barrier, and may be either organic or inorganic. Concretely, exemplified is the material disclosed in the Japanese Patent Application Laid-Open No. 8-248276, for example.
Electron Transport Layer
A material for the electron transport layer provided in accordance with requirements, which is only required to have a function of transporting to the organic light-emitting layer electrons applied from the cathode, may be selected from well-known materials. Concretely, exemplified is the material disclosed in the Japanese Patent Application Laid-open No. 8-248276, for example.
Further, each layer of the light emitting device can be formed in the well-known manner. For example, an optimal film-forming method can be selected for a material of each layer of the light emitting device. Concretely, vacuum evaporation, spin coating, LB or an inkjet method can be selected, for example.